The presence of sulfur moieties in petroleum feedstreams is highly undesirable since they can cause corrosion and environmental problems associated with end products, such as transportation fuels. Sulfur moieties can also affect the performance of engines using such fuels. Refined hydrocarbon streams are generally not transported in a pipeline previously used for the transportation of sour hydrocarbon streams, such as petroleum crudes, because the streams, such as gasoline and diesel fuels, can pick up contaminants from the pipeline, such as elemental sulfur. For example, about 10 to 80 mg/L of elemental sulfur is picked-up by gasoline and about 1 to 20 mg/L elemental sulfur is typically picked-up by diesel fuel when pipelined. Sulfur has a particularly corrosive effect on equipment, such as brass valves, gauges, silver bearing cages in two-cycle engines, and in-tank fuel pump copper commutators.
While maximum sulfur levels of 1000 wppm are found in some motor gasolines, government regulations will lead to sulfur levels of less than 30 wppm after 2003. Although significant changes in engine design have reduced total emissions, further decreases in level of sulfur emissions would be desirable.
Refiners have various options for producing low-sulfur gasoline. For example, they can refine relatively low sulfur crudes, or they can hydrotreat refinery streams to remove contaminants or use processes that include adsorption and absorption to remove contaminants. The world supply of low sulfur (sweet crude) is rapidly diminishing and, therefore, processing low sulfur crudes is not considered a long-term option.
Cracked naphthas, such as those derived from a fluidized catalytic cracking unit (FCCU), cokers and other high temperature cracking units have a high sulfur content compared to other gasoline blending components of the gasoline pool. A large portion of this sulfur is concentrated in the back end of the naphtha, i.e., heavy naphthas such as heavy cat naphtha. Therefore, reducing sulfur in gasoline could involve treating the feed and/or the products from a heavy naphtha process unit, such as a FCCU.
Gonzales et al. (“Can You Make Low-Sulfur Fuel and Remain Competitive,” Hart's Fuel Technology and Management, November/December 1996) indicates that cat feed desulfurization can reduce sulfur levels in cracked naphtha to about 500 wppm, or less. However, the cost of this option is generally balanced against the advantage of the higher gasoline conversions as a result of cat feed desulfurization. In another option, sulfur levels lower than about 200 wppm are achievable via non-selective hydrodesulfurizaton of light cracked naphtha. However, this can be incrementally more expensive than cat feed desulfurization because of the high hydrogen consumption and loss of octane due to the hydrogenation of olefins. Hydrotreated cracked-naphtha can be isomerized to recover some of the lost octane, but at additional cost. It is clear from the above information that there will be a significant cost associated with reducing the sulfur levels in gasoline, especially down to very low levels, such as 30 wppm.
Adsorption is often a cost-effective process to remove relatively low levels of contaminants. Salem, A. B. et al., “Removal of Sulfur Compounds from Naphtha Solutions Using Solid Adsorbents,” Chemical Engineering and Technology, Jun. 20, 1997, report a 65% reduction in the sulfur level (500 to 175 wppm) for a 50/50 mixture of virgin and cracked naphthas using activated carbon at 80° C. and a 30% reduction using Zeolite 13X at 80° C. Also, U.S. Pat. No. 5,807,475 teaches that Ni or Mo exchanged Zeolite X and Y can be used to remove sulfur compounds from hydrocarbon streams. Typical adsorption processes have an adsorption cycle whereby the contaminant is adsorbed from the stream followed by a desorption cycle whereby the adsorbent is regenerated by removing at least a portion, preferably substantially all, of the contaminants therefrom.
As with hydrotreating, adsorption will improve the stability of the gasoline product by removing unstable heteroatoms, such as nitrogen and sulfur contaminants.
Typically, the desorbed material produced during a conventional regeneration cycle contains a relatively high level of contaminants and is thus generally difficult and expensive to dispose of. Therefore, a regeneration cycle that produces a desorbed stream having relatively low levels of contaminants is highly desirable.